Ferroelectric memory cell recovery

ABSTRACT

Methods, systems, and devices for recovering fatigued ferroelectric memory cells are described. Recovery voltages may be applied to a ferroelectric memory cell that is fatigued due to repeated access (read or write) operations. The recovery voltage may have a greater amplitude than the access voltage and may include multiple voltage pulses or a constant voltage. The recovery operation may be performed in the background as the memory array operates, or it may be performed when a host device is not actively using the memory array. The recovery operations may be performed periodically or may include discrete series of pulses distributed among several instances.

CROSS REFERENCE

The present Application for Patent is a continuation of U.S. patentapplication Ser. No. 15/611,568 by Mariani et al., entitled“Ferroelectric Memory Cell Recovery,” filed Jun. 1, 2017, which is acontinuation of U.S. patent application Ser. No. 15/179,695 by Marianiet al., entitled “Ferroelectric Memory Cell Recovery,” filed Jun. 10,2016, assigned to the assignee hereof, and each of which is expresslyincorporated by reference in its entirety herein.

BACKGROUND

The following relates generally to memory devices and more specificallyto recovery of fatigued ferroelectric memory cells.

Memory devices are widely used to store information in variouselectronic devices such as computers, wireless communication devices,cameras, digital displays, and the like. Information is stored byprogramming different states of a memory device. For example, binarydevices have two states, often denoted by a logic “1” or a logic “0.” Inother systems, more than two states may be stored. To access the storedinformation, the electronic device may read, or sense, the stored statein the memory device. To store information, the electronic device maywrite, or program, the state in the memory device.

Various types of memory devices exist, including random access memory(RAM), read only memory (ROM), dynamic RAM (DRAM), synchronous dynamicRAM (SDRAM), ferroelectric RAM (FeRAM), magnetic RAM (MRAM), resistiveRAM (RRAM), flash memory, and others. Memory devices may be volatile ornon-volatile. Non-volatile memory, e.g., flash memory, can store datafor extended periods of time even in the absence of an external powersource. Volatile memory devices, e.g., DRAM, may lose their stored stateover time unless they are periodically refreshed by an external powersource. A binary memory device may, for example, include a charged ordischarged capacitor. A charged capacitor may, however, becomedischarged over time through leakage currents, resulting in the loss ofthe stored information. Certain features of volatile memory may offerperformance advantages, such as faster read or write speeds, whilefeatures of non-volatile memory, such as the ability to store datawithout periodic refreshing, may be advantageous.

FeRAM may use similar device architectures as volatile memory but mayhave non-volatile properties due to the use of a ferroelectric capacitoras a storage device. FeRAM devices may thus have improved performancecompared to other non-volatile and volatile memory devices. Theperformance of FeRAM cells may degrade over their lifetime, however. Forexample, the ferroelectric material may experience fatigue due to theread or write operations performed on the memory cell during normaloperation. A fatigued ferroelectric material may reduce the FeRAM cell'sability to store charge, which may make the memory cell inoperable.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure herein refers to and includes the following figures:

FIG. 1 illustrates an example memory array that supports recovery offatigued ferroelectric memory cells in accordance with variousembodiments of the present disclosure;

FIG. 2 illustrates an example circuit of a memory cell that supportsrecovery from fatigue in accordance with various embodiments of thepresent disclosure;

FIG. 3 illustrates example hysteresis plots for a pristine and afatigued ferroelectric memory cells in accordance with variousembodiments of the present disclosure;

FIG. 4 illustrates example recovery operations that support recovery offatigued ferroelectric memory cells in accordance with variousembodiments of the present disclosure;

FIG. 5 illustrates example distributed operations that support recoveryof fatigued ferroelectric memory cells in accordance with variousembodiments of the present disclosure;

FIG. 6 illustrates a block diagram of an example ferroelectric memoryarray that supports recovery of fatigued ferroelectric memory cells inaccordance with various embodiments of the present disclosure;

FIG. 7 illustrates a block diagram of an example ferroelectric memoryarray that supports recovery of fatigued ferroelectric memory cells inaccordance with various embodiments of the present disclosure;

FIG. 8 illustrates a system, including a memory array, that supportsrecovery of fatigued ferroelectric memory cells in accordance withvarious embodiments of the present disclosure; and

FIG. 9 is a flowchart that illustrates a method or methods for recoveryof fatigued ferroelectric memory cells in accordance with variousembodiments of the present disclosure.

DETAILED DESCRIPTION

Fatigued ferroelectric memory cells, including those suffering fromcycling-induced fatigue, may be recovered by applying a recovery voltageto the memory cells. The recovery voltage may have a greater amplitudethan the cycling voltage (e.g., the voltages used to read or write amemory cell). The recovery voltages may include multiple voltage pulsesor a constant voltage stress. In order to minimize interruptions of ahost device's operation, the recovery operation may be applied duringtimes when the memory device is not in use by the host device, and therecovery operation may be distributed, or broken up, over time. Forexample, the recovery operation may be performed while the host deviceis powering on or off, or it may occur in the background while thememory array is idle.

Cycling-induced fatigue may decrease a ferroelectric memory cell'sremnant polarization, which is the polarization that remains afterreading or writing the memory cell. Since the charge stored in aferroelectric memory cell is proportional to its remnant polarization,less charge is stored in the memory cell as the remnant polarizationdecreases. If the remnant polarization drops below a threshold, thememory array may not be able to read the logic value of the fatiguedmemory cell. That is, the memory array may not be sufficiently sensitiveto determine the logic state based on the decreased charge. As a result,the memory cell may be considered inoperable or dead.

A recovery operation may counteract fatigue by restoring the remnantpolarization of the memory cell. For example, an applied recoveryvoltage may improve or restore the ferroelectric material's remnantpolarization. The effectiveness of the recovery operation may depend onthe amplitude of the recovery voltage and its duration. In someexamples, the recovery operation may include bipolar voltage pulses,unipolar voltage pulses, or a constant voltage.

Features of the disclosure introduced above are further described belowin the context of a memory array. Specific examples are then describedfor recovery of fatigued ferroelectric memory cells and variousimplementations of such recovery operations. These and other features ofthe disclosure are further illustrated by and described with referenceto apparatus diagrams, system diagrams, and flowcharts that relate torecovery of fatigued ferroelectric memory cells.

FIG. 1 illustrates an example memory array 100 that supports recovery offatigued ferroelectric memory cells in accordance with variousembodiments of the present disclosure. Memory array 100 may also bereferred to as an electronic memory apparatus. Memory array 100 includesmemory cells 105 that are programmable to store different states. Eachmemory cell 105 may be programmable to store two states, denoted as alogic 0 and a logic 1. In some cases, memory cell 105 is configured tostore more than two logic states. A memory cell 105 may include acapacitor to store a charge representative of the programmable states;for example, a charged and uncharged capacitor may represent two logicstates, respectively. Memory cell 105 may include a capacitor with aferroelectric material. Ferroelectric materials have a spontaneouselectric polarization—i.e., they have a non-zero polarization in theabsence of an electric field. Some details and advantages of aferroelectric memory cell 105 are discussed below. Different levels ofcharge of a ferroelectric capacitor may represent different logicstates. As a ferroelectric memory cell 105 is cycled (e.g., cycledthrough read or write operations), its polarization may decrease due tofatigue, reducing the stored charge. Recovery operations may be appliedto the memory cell 105 in order to restore its polarization.

Operations such as reading and writing (i.e., cycling) may be performedon memory cells 105 by activating or selecting the appropriate accessline 110 and digit line 115. Access lines 110 may also be referred to asword lines 110 and digit lines 115 may also be referred to as bit lines115. Activating or selecting a word line 110 or a digit line 115 mayinclude applying a voltage to the respective line. Word lines 110 anddigit lines 115 are made of conductive materials. For example, they maybe made of metals (such as copper, aluminum, gold, tungsten, etc.),metal alloys, other conductive materials, or the like. According to theexample of FIG. 1, each row of memory cells 105 is connected to a singleword line 110, and each column of memory cells 105 is connected to asingle digit line 115. By activating one word line 110 and one digitline 115 (e.g., applying a voltage to the word line 110 or digit line115), a single memory cell 105 may be accessed at their intersection,which may be referred to as memory cell's address. Each access operationmay reduce the remnant polarization of a memory cell 105 and, as thenumber of access operations increases with use, a memory cell 105 maybecome inoperable due to fatigue.

In some architectures, the logic storing device of a cell, e.g., acapacitor, may be electrically isolated from the digit line by aselection component. The word line 110 may be connected to and maycontrol the selection component. For example, the selection componentmay be a transistor and the word line 110 may be connected to the gateof the transistor. Activating the word line 110 results in an electricalconnection or closed circuit between the capacitor of a memory cell 105and its corresponding digit line 115. The digit line may then beaccessed to either read or write the memory cell 105. Otherarchitectures may be used that do not include a transistor as aselection device. For example, a cross-point memory array architecturemay be used.

Accessing memory cells 105 may be controlled through a row decoder 120and a column decoder 130. In some examples, a row decoder 120 receives arow address from the memory controller 140 and activates the appropriateword line 110 based on the received row address. Similarly, a columndecoder 130 receives a column address from the memory controller 140 andactivates the appropriate digit line 115. For example, memory array 100may include multiple word lines 110, labeled WL_1 through WL_M, andmultiple digit lines 115, labeled DL_1 through DL_N, where M and Ndepend on the array size. Thus, by activating a word line 110 and adigit line 115, e.g., WL_2 and DL_3, the memory cell 105 at theirintersection may be accessed. In some cases, the recovery operation maybe applied to a row, column, some combination of rows or columns, or theentire array. In other cases, memory array 100 may be a memory bank, andthe recovery operation may be applied to the memory bank.

Upon accessing, a memory cell 105 may be read, or sensed, by sensecomponent 125 to determine the stored state of the memory cell 105. Forexample, after accessing the memory cell 105, the ferroelectriccapacitor of memory cell 105 may discharge onto its corresponding digitline 115. Discharging the ferroelectric capacitor may be based onbiasing, or applying a voltage, to the ferroelectric capacitor. Thedischarging may induce a change in the voltage of the digit line 115,which sense component 125 may compare to a reference voltage (not shown)in order to determine the stored state of the memory cell 105. Forexample, if digit line 115 has a higher voltage than the referencevoltage, then sense component 125 may determine that the stored state inmemory cell 105 was a logic 1 and vice versa. Sense component 125 mayinclude various transistors or amplifiers in order to detect and amplifya difference in the signals, which may be referred to as latching. Thedetected logic state of memory cell 105 may then be output throughcolumn decoder 130 as output 135. In some cases, the sense operation maydetermine that the memory cell 105 has reached a fatigue threshold. Forexample, sense component 125 may determine a reduction in stored chargeof the memory cell 105 due to fatigue. A recovery operation may then beperformed on the memory cell 105.

A memory cell 105 may be set, or written, by activating the relevantword line 110 and digit line 115. As discussed above, activating a wordline 110 electrically connects the corresponding row of memory cells 105to their respective digit lines 115. By controlling the relevant digitline 115 while the word line 110 is activated, a memory cell 105 may bewritten—i.e., a logic value may be stored in the memory cell 105. Columndecoder 130 may accept data, for example input 135, to be written to thememory cells 105. A ferroelectric memory cell 105 may be written byapplying a voltage across the ferroelectric capacitor. The recoveryvoltages may, in some examples, be applied in a similar manner. That is,the recovery voltage may be applied across the ferroelectric capacitorby activating the relevant word line 110 and digit line 115. In someexamples, the recovery voltage may be applied using a plate line, asdiscussed in more detail below.

In some memory architectures, such as DRAM, a single access operation ofthe memory cell 105 may degrade or destroy its stored logic state, andre-write or refresh operations may be performed to return the originallogic state to memory cell 105. In DRAM, for example, the capacitor maybe partially or completely discharged during a sense operation,corrupting the stored logic state. So the logic state may be re-writtenafter a sense operation. Additionally, activating a single word line 110may result in the discharge of all memory cells in the row; thus,several or all memory cells 105 in the row may need to be re-written.

Some memory architectures, including DRAM, may lose their stored stateover time unless they are periodically refreshed by an external powersource. For example, a charged capacitor may become discharged over timethrough leakage currents, resulting in the loss of the storedinformation. The refresh rate of these so-called volatile memory devicesmay be relatively high, e.g., tens of refresh operations per second forDRAM arrays, which may result in significant power consumption. Withincreasingly larger memory arrays, increased power consumption mayinhibit the deployment or operation of memory arrays (e.g., powersupplies, heat generation, material limits, etc.), especially for mobiledevices that rely on a finite power source, such as a battery. Asdiscussed below, ferroelectric memory cells 105 may have beneficialproperties that may result in improved performance relative to othermemory architectures. By performing recovery operations to ferroelectricmemory cells 105, their lifetime may be extended.

The memory controller 140 may control the operation (e.g., read, write,re-write, refresh, recovery, etc.) of memory cells 105 through thevarious components, such as row decoder 120, column decoder 130, andsense component 125. Memory controller 140 may generate row and columnaddress signals in order to activate the desired word line 110 and digitline 115. Memory controller 140 may also generate and control variousvoltages used during the operation of memory array 100. In general, theamplitude, shape, or duration of an applied voltage discussed herein maybe adjusted or varied and may be different for the various operationsfor operating memory array 100. Furthermore, one, multiple, or allmemory cells 105 within memory array 100 may be accessed simultaneously;for example, multiple or all cells of memory array 100 may be accessedsimultaneously during a reset operation in which all memory cells 105,or a group of memory cells 105, are set to a single logic state. Memorycontroller 140 may also distribute the recovery operations over multipleinstances, which may help prevent the disruption of the host device'soperation.

FIG. 2 illustrates an example circuit 200 that includes a memory cell105 and supports recovery of fatigued ferroelectric memory cells inaccordance with various embodiments of the present disclosure. Circuit200 includes a ferroelectric memory cell 105-a, word line 110-a, digitline 115-a, and sense component 125-a, which may be examples of a memorycell 105, word line 110, digit line 115, and sense component 125,respectively, as described with reference to FIG. 1. Memory cell 105-amay include a logic storage component, such as capacitor 205 that has afirst plate, cell plate 230, and a second plate, cell bottom 215. Cellplate 230 and cell bottom 215 may be capacitively coupled through aferroelectric material positioned between them. The orientation of cellplate 230 and cell bottom 215 may be flipped without changing theoperation of memory cell 105-a. Circuit 200 also includes selectioncomponent 220 and reference signal 225. In the example of FIG. 2, cellplate 230 may be accessed via plate line 210 and cell bottom 215 may beaccessed via digit line 115-a. As described above, various states may bestored by charging or discharging capacitor 205. Recovery operations maybe applied to memory cell 105-a, for example, by applying a voltageacross the ferroelectric capacitor 205 using plate line 210 or digitline 115-a.

The stored state of capacitor 205 may be read or sensed by operatingvarious elements represented in circuit 200. Capacitor 205 may be inelectronic communication with digit line 115-a. For example, capacitor205 can be isolated from digit line 115-a when selection component 220is deactivated, and capacitor 205 can be connected to digit line 115-awhen selection component 220 is activated. Activating selectioncomponent 220 may be referred to as selecting memory cell 105-a. In somecases, selection component 220 is a transistor and its operation iscontrolled by applying a voltage to the transistor gate, where thevoltage magnitude is equal to or greater than the magnitude of thethreshold of the transistor. Word line 110-a may activate selectioncomponent 220; for example, a voltage applied to word line 110-a isapplied to the transistor gate, connecting capacitor 205 with digit line115-a. In an alternative embodiment, the positions of selectioncomponent 220 and capacitor 205 may be switched, such that selectioncomponent 220 is connected between plate line 210 and cell plate 230 andsuch that capacitor 205 is between digit line 115-a and the otherterminal of selection component 220. In this embodiment, selectioncomponent 220 may remain in electronic communication with digit line115-a through capacitor 205. This configuration may be associated withalternative timing and biasing for read and write operations.

Due to the ferroelectric material between the plates of capacitor 205,and as discussed in more detail below, capacitor 205 may not dischargeupon connection to digit line 115-a. To sense the logic state stored byferroelectric capacitor 205, word line 110-a may be biased to selectmemory cell 105-a and a voltage may be applied to plate line 210. Thisbias may be applied after activating selection component 220, or thebias may be constantly applied to cell plate 230. Biasing plate line 210may result in a voltage difference across capacitor 205, which may yielda change in the stored charge on capacitor 205. The magnitude of thechange in stored charge may depend on the initial state of capacitor205—e.g., whether the initial state stored a logic 1 or a logic 0. Thismay induce a change in the voltage of digit line 115-a based on thecharge stored on capacitor 205, which may be used to determine thestored logic state

The change in the voltage of digit line 115-a may depend on itsintrinsic capacitance—e.g., as digit line 115-a is energized, somefinite charge may be stored in digit line 115-a and the resultingvoltage of the digit line may depend on the intrinsic capacitance ofdigit line 115-a. The intrinsic capacitance may depend on physicalcharacteristics, including the dimensions, of digit line 115-a. Digitline 115-a may connect many memory cells 105 so digit line 115-a mayhave a length that results in a non-negligible capacitance (e.g., on theorder of picofarads (pF)). The resulting voltage of digit line 115-a maythen be compared to a reference signal 225 (e.g., a voltage of referenceline) by sense component 125-a in order to determine the stored logicstate in memory cell 105-a. As memory cell 105-a fatigues, the resultingvoltage of digit line 115-a may change because less charge may be storedin memory cell 105-a. As such, the resulting digit line 115-a voltagemay be used to determine if memory cell 105-a has reached its fatiguethreshold.

Sense component 125-a may include various transistors or amplifiers todetect and amplify a difference in signals, which may be referred to aslatching. Sense component 125-a may include a sense amplifier thatreceives and compares the voltage of digit line 115-a and referencesignal 225, which may be a reference voltage. Sense component 125-a maythen latch the output of the sense amplifier or the voltage of digitline 115-a, or both. The latched logic state of memory cell 105-a maythen be output, for example, through column decoder 130 as output 135with reference to FIG. 1.

To write memory cell 105-a, a voltage may be applied across capacitor205. Various methods may be used. In some examples, selection component220 may be activated through word line 110-a in order to electricallyconnect capacitor 205 to digit line 115-a. For a ferroelectric capacitor205, a voltage may be applied across capacitor 205 by controlling thevoltage of cell plate 230 (through plate line 210) and cell bottom 215(through digit line 115-a) to apply a positive or negative voltageacross the capacitor 205.

Recovery operations may also apply voltages across capacitor 205. Forexample, selection component 220 may be activated and a voltage may beapplied across capacitor 205 using plate line 210 and digit line 115-a.In some examples, plate line 210 may be connected to multiple memorycells 105, and the recovery operation may be applied to each memory cell105 connected to plate line 210. For example, plate line 210 may becommonly connected to a row of memory cells 105 and energizing word line110-a may select all memory cells 105 in the corresponding row, asdepicted in FIG. 1. Thus, multiple memory cells 105 may be recovered byenergizing a single word line 110-a and plate line 210.

A recovery operation may use voltage amplitudes greater than those usedduring read or write operations. In some cases, internal circuitry maybe used to create the larger recovery voltages. In other cases, plateline 210 may be in electronic communication with a voltage source usedfor recovery that is different from a voltage source used for memorycell access operations.

FIG. 3 illustrates example hysteresis plots for a ferroelectric memorycell that supports fatigue recovery of in accordance with variousembodiments of the present disclosure. Hysteresis plots 300 depict thecharge, Q, stored on a ferroelectric capacitor (e.g., capacitor 205 ofFIG. 2) as a function of a voltage difference, V. The charge isproportional to the polarization of the ferroelectric material.Hysteresis plot 300-a shows example write operations of a ferroelectricmemory cell 105, and hysteresis plot 300-b compares a pristine andfatigued ferroelectric memory cell 105.

A ferroelectric material is characterized by a spontaneous electricpolarization, i.e., it maintains a non-zero electric polarization in theabsence of an electric field. Example ferroelectric materials includebarium titanate (BaTiO₃), lead titanate (PbTiO₃), lead zirconiumtitanate (PZT), and strontium bismuth tantalate (SBT). The ferroelectriccapacitors described herein may include these or other ferroelectricmaterials. Electric polarization within a ferroelectric capacitorresults in a net charge at the ferroelectric material's surface andattracts opposite charge through the capacitor terminals. Thus, chargeis stored at the interface of the ferroelectric material and thecapacitor terminals. Because the electric polarization may be maintainedin the absence of an externally applied electric field for relativelylong times, even indefinitely, charge leakage may be significantlydecreased as compared with, for example, capacitors employed in DRAMarrays. This may reduce the need to perform refresh operations asdescribed above for some DRAM architectures.

Hysteresis plots 300 may be understood from the perspective of a singleterminal of a capacitor. By way of example, if the ferroelectricmaterial has a negative polarization, positive charge accumulates at theterminal. Likewise, if the ferroelectric material has a positivepolarization, negative charge accumulates at the terminal. Additionally,it should be understood that the voltages in hysteresis plots 300represent a voltage difference across the capacitor and are directional.For example, a positive voltage may be realized by applying a positivevoltage to the terminal in question (e.g., a cell plate 230) andmaintaining the second terminal (e.g., a cell bottom 215) at ground (orapproximately zero volts (0V)). A negative voltage may be applied bymaintaining the terminal in question at ground and applying a positivevoltage to the second terminal—i.e., positive voltages may be applied tonegatively polarize the terminal in question. Similarly, two positivevoltages, two negative voltages, or any combination of positive andnegative voltages may be applied to the appropriate capacitor terminalsto generate the voltage difference shown in hysteresis plots 300.

As depicted in hysteresis plot 300-a, the ferroelectric material maymaintain a positive or negative polarization with a zero voltagedifference, resulting in two possible charged states: charge state 305and charge state 310. According to the example of FIG. 3, charge state305 represents a logic 0 and charge state 310 represents a logic 1. Insome examples, the logic values of the respective charge states may bereversed without loss of understanding or operation.

A logic 0 or 1 may be written to the memory cell by controlling theelectric polarization of the ferroelectric material, and thus the chargeon the capacitor terminals, by applying voltage. For example, applying anet positive voltage 315 across the capacitor results in chargeaccumulation until charge state 305-a is reached. Upon removing voltage315, charge state 305-a follows path 320 until it reaches charge state305 at zero voltage potential. Similarly, charge state 310 is written byapplying a net negative voltage 325, which results in charge state310-a. After removing negative voltage 325, charge state 310-a followspath 330 until it reaches charge state 310 at zero voltage. Chargestates 305 and 310 may also be referred to as the remnant polarization(Pr) values, i.e., the polarization (or charge) that remains uponremoving the external bias (e.g., voltage).

To read, or sense, the stored state of the ferroelectric capacitor, avoltage may be applied across the capacitor. In response, the storedcharge, Q, changes, and the degree of the change depends on the initialcharge state—i.e., the final stored charge (Q) depends on whether chargestate 305-b or 310-b was initially stored. In some cases, the finalstored charge may alter the voltage of the digit line in electroniccommunication with the memory cell 105. By comparing the digit linevoltage to a reference voltage, the initial state of the capacitor maybe determined.

In some cases, a read operation may be followed by a write-backoperation, where the originally stored logic value is written to thememory cell 105. That is, the read operation may destroy the originallystored logic value of the target memory cell 105. The read process mayuse a positive voltage, for example, voltage 315 may be applied to thememory cell 105, although other voltages may be used. If a logic 1 wasoriginally stored, the read voltage may result in charge state 310following hysteresis plot 300-a until it reaches, for example, chargestate 305-a, although other positions may be possible depending theexact sensing scheme. After the read voltage is removed, the chargestate may not return to its original state, charge state 310, rather itmay follow a different path, for example, path 320 and settle at chargestate 305. In other words, a read operation of a logic 1 may result inwriting a logic 0 to the memory cell. Thus, a write-back operation maybe performed to return the originally stored logic value to the memorycell. For example, a negative voltage, such as voltage 325, may beapplied to write-back the original logic 1 value.

As mentioned above, the remnant polarization may decrease with thenumber of cycles or access operations applied to a ferroelectric memorycell 105. Hysteresis plot 300-b compares a pristine and fatiguedferroelectric memory cell 105. Pristine hysteresis plot 335 (dashedline) shows the hysteresis plot for a pristine ferroelectric memory cell105, whereas fatigued hysteresis plot 340 (solid line) shows ahysteresis plot for a fatigued memory cell 105. The fatigued hysteresisplot has a lower remnant polarization (indicated by charge states 305-band 310-b) compared to the remnant polarization of pristine hysteresisplot 335 (indicated by charge states 305-a and 310-a).

Each access operation may further fatigue the memory cell; that is, bothcharge states 305-b and 310-b may continue to decrease in magnitude.This may affect the ability to sense or read the stored logic state inthe memory cell 105. Reading a memory cell 105 may includedifferentiating between the two charge states, 305 and 310. As theferroelectric capacitor continues to fatigue, the separation betweencharge states 305 and 310 continues to decrease. At some point, thememory array (e.g., a controller, sense amplifier, etc.) may not be ableto properly distinguish between the two charge states—i.e., a minimumseparation between charge states 305 and 310 may be needed to read thestored logic state. For example, the difference between charge states305-b and 310-b may be insufficient for a sense component 125 to readthe memory cell 105. In some cases, depending on the ferroelectricmaterial, the fatigue limit may occur when the cycle counts exceedsapproximately 10⁸ cycles, however, it may depend on the sensing schemeused by the memory array. For example, it may not occur until 10¹³cycles.

A recovery operation may improve or restore the remnant polarization andthus increase the stored charge. For example, applying a recoveryvoltage to a fatigued memory cell 105 may restore fatigued hysteresisplot 340 to pristine hysteresis plot 335, thereby increasing the remnantpolarization from charge states 305-b and 310-b to 305-a and 310-a,respectively. In some cases, the recovery operation may be appliedbefore the fatigue threshold is reached, for example, at 10¹⁰ cycles.The recovery operation may include significantly fewer cycles, forexample, 10⁴-10⁸, although other values are possible.

FIG. 4 illustrates example recovery operations, using plots 400-a,400-b, and 400-c, that support recovery of fatigued ferroelectric memorycells in accordance with various embodiments of the present disclosure.Plots 400 illustrate example voltage cycling of a ferroelectric memorycell 105 with time. Plots 400 include access operations 405, recoveryoperation 410, and recovery measurement operation 415.

A memory cell 105 may be cycled—through reading or writing—duringoperation of the memory device. After some threshold (e.g., time, numberof cycles, charge sensing, among others), recovery operation 410 may beperformed to improve the remnant polarization of the memory cell 105, asdiscussed above with reference to FIG. 3. After the recovery operation,recovery measurement operation 415 may determine the effectiveness ofthe recovery operation 410. In some cases, recovery measurementoperation 415 may not be performed; that is, normal cycling may beginafter recovery operation 410. In other cases, recovery measurementoperation 415 may be a single access operation of the memory cell 105.

Each operation—access operations 405, recovery operation 410, recoverymeasurement operation 415—may be applied to one or more memory cells105. For example, the operations may represent the respective operationsapplied to a row, column, or various row/column combinations of memorycells 105. In some cases, the operations may be performed on an entirememory array 100. During access operations 405, a first voltage may beapplied to a ferroelectric memory cell 105 during each cycle of aplurality of access cycles. For example, applying the first voltage mayinclude reading or writing the ferroelectric memory cell 105.

After some number of access operations, it may be determined that theferroelectric memory cell 105 reached a fatigue threshold based onapplying the first voltage during access operations 405. The fatiguethreshold may be based on a total number of access cycles in which theferroelectric memory cell 105 reaches a remnant polarization threshold,as discussed above with reference to FIG. 3.

In some cases, determining that the memory cell 105 reached the fatiguethreshold may include detecting one of a number of possible events. Forexample, detecting a timer exceeding a threshold time period, where thethreshold time period may be based on a time to reach the fatiguethreshold. In some cases, the time to reach the fatigue threshold may bebased on a duration of each access cycle (e.g., a time period based onaverage operation of the memory device or a worst-case scenario ofconstant access operations).

In another example, determining that the memory cell 105 reached thefatigue threshold may include determining that a counter exceeded athreshold number of counts, where the counter is incremented for eachaccess cycle of the plurality of access cycles. In some cases, thethreshold number of counts is based on a number of access cycles toachieve the fatigue threshold of the ferroelectric memory cell 105. Insome cases, this may be predetermined or programmed by a user.

In another example, determining that the memory cell 105 reached thefatigue threshold may include detecting a failure to sense the memorycell 105 based on applying a test access voltage with an amplitude lessthan an amplitude of the first voltage during access operations 405. Forexample, a lower access voltage may be applied to the ferroelectricmemory cell 105 (e.g., using a plate line 210), which may result in adigit line 115 having a lower voltage, as discussed with reference toFIG. 3. Due to the lower digit line 115 voltage, the sense component 125may be unable to read the memory cell 105, and recovery operations 410may be initiated. That is, the fatigue threshold of the ferroelectricmemory cell 105 may be based on a relationship between the charge storedin the ferroelectric memory cell 105 and a capacitance of the digit line115 in electronic communication with the ferroelectric memory cell 105and the sense component 125. By using a test voltage with a amplitudeless than the typical access voltage, the memory cell 105 may berestored before it fails due to fatigue, preventing interruption of thememory device's operation.

In some examples, determining that the memory cell 105 reached thefatigue threshold may include detecting a failure to sense theferroelectric memory cell 105 based on a test sense window that is lessthan a sense window used for each cycle of the plurality of accesscycles. For example, the test sense window may be based on a referencevoltage, and a different reference voltage may be used to modify thesize of the test sense window.

In some examples, an error correction code that detects a corruptedlogic value of the ferroelectric memory cell 105 may trigger thedetermination that memory cell 105 reached its fatigue threshold. Forexample, the sense component 125 may not be able to read the memory cell105 and the unknown logic value may be corrected by the error correctioncode. Other events may be possible to determine if the memory cell 105reached its fatigue threshold. For example, a command to initiate thefatigue recovery operation may be received. In some cases, the commandmay be generated external to the memory array, such as a system request.

After determining that ferroelectric memory cell 105 reached its fatiguethreshold, a second voltage may be applied to the ferroelectric memorycell 105—for example, during recovery operations 410. In some cases, anamplitude of the second voltage may be greater than an amplitude of thefirst voltage. The second voltage may be applied for a time period or anumber of repetitions that is based on the fatigue threshold of theferroelectric memory cell. In some examples, the time period or thenumber of repetitions of the second voltage is based on the amplitude ofthe second voltage. In some cases, a counter may be incremented eachtime the second voltage is applied during recovery operations 410.

In some examples, the second voltage may be applied during a refreshoperation that restores a logic state to the ferroelectric memory cell105, where the logic state is determined based on applying the firstvoltage. For example, a read operation during an access operation 405may be followed by a write-back or refresh operation. The secondvoltage—the recovery operation 410—may be applied during the write-backoperation. In some examples, several cycles at the higher voltage or aconstant voltage stress may be applied during the refresh operation.

In some cases, the memory array 100 comprises an element of a device,for example, a mobile device or any other electronic device. The secondvoltage used during recovery operations 410 may be applied to theferroelectric memory cell 105 during an event that includes at least oneof the device powering on, the device powering off, or the device beingconnected to an external power supply. Or, memory array 100 may receivea command, for example from another element of the device, to initiatethe recovery operation and apply the second voltage. This may reduce theeffect of recovery operations 410 on the operation of the host device orits power source (e.g., its battery). In some cases, recovery operations410 may be a burst of recovery cycles performed at a fixed rate—forexample, when the host device is charging and connected to an externalpower source.

In some examples, recovery operation 410 may be applied to the entirememory array. In other cases, recovery operation 410 may be applied to asubset of the memory array. For example, part of the memory array may berecovered while the remainder of the array operates normally, such thatthe recovery operation 410 can occur in the background without affectingthe host device's operation.

After performing recovery operations 410, recovery measurement operation415 may be performed. For example, recovery measurement operation 415may determine if the recovery operations 410 restored the memory cell105 from fatigue effects. In some cases, this may include a test accessoperation, e.g., a read operation. Recovery measurement operation 415may determine that a charge stored in the ferroelectric memory cell 105after applying the second voltage (i.e., during recovery operations 410)is greater than a charge stored in the memory cell 105 before applyingthe second voltage. In some cases, the determination may be based on asense voltage produced by the memory cell 105, as described withreference to FIG. 3.

Plots 400-a, 400-b, and 400-c illustrate different recovery operations410. Plot 400-a illustrates a recovery operation 410 where the secondvoltage is a plurality of bipolar voltage pulses. In other cases, thesecond voltage may be a plurality of unipolar pulses, as shown inrecovery operation 410-a of plot 400-b. In other examples, such asrecovery operation 410-b of plot 400-c, the second voltage may be avoltage with a constant amplitude. Although shown with positiveamplitudes in plots 400-b and 400-c, recovery operations 410-a and 410-bmay use negative amplitudes. The number of recovery pulses may vary, forexample, 10⁴ to 10⁸ cycles may be used, although other values may bepossible. The time period of recovery operation 400-b may be equal tothe total time period in which the voltage pulses of recovery operations410 and 410-a are applied.

The effect of recovery operation 410 may depend on the amplitude of thesecond voltage and the number of pulses or the total time length of theconstant voltage. That is, a pulse with a larger amplitude may achieverecovery with fewer pulses or a shorter time period. For example, amemory cell 105 fatigued by 1.5V access operations may be recovered by10⁷ pulses using a 1.8V recovery voltage amplitude. In another example,a memory cell 105 fatigued by 1.5V access operations may be recovered by10⁴ pulses using a 2.4V amplitude. Other access and recovery amplitudesare possible, as well as the number of pulses.

In some examples, recovery operation 410 may be more effective with ahigher number of recovery cycles. In other examples, slower recoverypulses may improve the recovery effect compared to faster pulses. Therecovery effectiveness may be a function of the total duration of therecovery cycles (recovery operations 410 and 410-a) or the constantamplitude (recovery operation 410-b).

In some examples, recovery operations 410 may be changed during theoperation of the memory device. For example, the amplitude of recoveryoperations 410 may change over the memory cell's 105 lifetime. Forexample, later recovery operations 410 may use a greater amplitude torecover a memory cell 105. In some cases, the number of pulses or thetime period of the second voltage may change as well, for example, morerecovery pulses may be used later in the memory cell's lifetime.

FIG. 5 illustrates an example distributed recovery operation, using plot500, that supports recovery of fatigued ferroelectric memory cells inaccordance with various embodiments of the present disclosure. Plot 500illustrates example voltage cycling of a ferroelectric memory cell 105with time, which may be an example of a plot 400 with reference to FIG.4. Plot 500 include access operations 405-c and 405-d and recoveryoperations 410-c, 410-d, and 410-e, which may be examples of accessoperations 405 and recovery operations 410, as discussed with referenceto FIG. 4. Recovery operations 410-c, 410-d, and 410-e may use anycombination of unipolar pulses, bipolar pulses, or constant amplitudevoltages, as discussed with reference to FIG. 4. As illustrated,recovery operations 410 may be distributed during the operation of thememory cell 105. For example, recovery operations 410 may beinterrupted, for example, due to the host device using the memory array,and the recovery operations 410 may be paused. Recovery operations mayalso be applied or performed periodically. Each access or recoveryoperation may be applied to a row, column, or various row/columncombinations of memory cells 105, including an entire memory array 100.In some cases, a recovery measurement operation 415 may be appliedfollowing a recovery operation 410, as discussed with reference to FIG.4.

During access operations 405-c, a first voltage may be applied to aferroelectric memory cell 105 during each cycle of a plurality of accesscycles. For example, applying the first voltage to access theferroelectric memory cell 105 may include reading or writing theferroelectric memory cell 105.

After some number of access operations, the ferroelectric memory cell105 may be determined to have reached a fatigue threshold based onapplying the first voltage during access operations 405-c. In somecases, determining that the ferroelectric memory cell 105 has reachedthe fatigue threshold may include detecting one of a number of possibleevents as described with reference to FIG. 4.

After determining that ferroelectric memory cell 105 reached its fatiguethreshold, a second voltage may be applied to the ferroelectric memorycell 105 during recovery operations 410-c. In some cases, an amplitudeof the second voltage may be greater than an amplitude of the firstvoltage. The second voltage may be applied for a time period or a numberof repetitions that is based on the fatigue threshold of theferroelectric memory cell. In some examples, the time period or thenumber of repetitions of the second voltage is based on the amplitude ofthe second voltage.

In some cases, the recovery operation 410 may be distributed amongmultiple recovery operations, such as recovery operations 410-c, 410-d,and 410-e. That is, the second voltage may be applied for the timeperiod or the number of repetitions during a plurality of instances ofan event, where a subset of the time period or a subset of the number ofrepetitions is associated with each instance of the event. For example,recovery operations 410-c, 410-d, and 410-e may represent the pluralityof instances of the event. In some examples, the memory array comprisesan element of a device, and the event may include at least one of thedevice powering on, the device powering off, or the device beingconnected to an external power supply. Or the recovery operation 410 maybe performed after receiving a command to perform the recoveryoperation, for example, receiving an external command.

The memory device may operate normally between each recovery operation410. For example, access operations 405-d may represent the operation ofthe memory device during the time period after the device powered on butis not currently powering down. Or, access operations 405-d mayrepresent operation when the device is not connected to an externalpower supply.

In some cases, access operations 405-d may represent a command beingreceived by the memory array to access the array's contents. Forexample, the application of the second voltage may be suspended based onreceiving the request to access the memory array. After the accessoperation has completed, recovery operations may resume as shown withrecovery operations 410-e. That is, application of the second voltagemay be resumed based on completing the request.

During recovery operations 410-c, 410-d, and 410-e, a counter may beincremented each time the second voltage is applied. For example, thetotal recovery operation may include some predetermined number ofpulses, and the counter may keep track of the total number of appliedpulses throughout the distributed recovery operation 410.

FIG. 6 illustrates a block diagram 600 of a memory array that supportsrecovery of fatigued ferroelectric memory cells in accordance withvarious embodiments of the present disclosure. Memory array 100-a may bereferred to as an electronic memory apparatus and includes memorycontroller 140-a and memory cell 105-b, which may be examples of memorycontroller 140 and memory cell 105 described with reference to FIGS. 1and 2. Memory cells 105-b may be ferroelectric memory cells. Memoryarray 100-a includes counter 605, timer 610, and cache 615. Cache 615may include memory cells 105, which may be memory cells of any type, forexample, non-volatile or volatile, such as DRAM cells.

Memory array 100-a may include multiple ferroelectric memory cells105-b. Counter 605 may be resettable based on a fatigue recoveryoperation performed on at least one ferroelectric memory cell 105 of theplurality of ferroelectric memory cells 105-b. In other cases, timer 610may be reset instead of counter 605. In some cases, memory array 100-amay include a plurality of memory blocks that comprise a subset of thememory array, where each memory block may be associated with at leastone counter 605 or timer 610.

Memory controller 140-a may be configured to perform the fatiguerecovery operation. The fatigue recovery operation may include adetermination that the at least one ferroelectric memory cell 105-b hasreached a fatigue threshold based on applying a first voltage to the atleast one ferroelectric memory cell 105-b for a plurality of accesscycles. In some examples, memory controller 140 may receive a commandinstructing it to perform the recovery operation. The fatigue recoveryoperation may also include applying a second voltage to the at least oneferroelectric memory cell 105-b based on the determination that theferroelectric memory cell 105-b has reached the fatigue threshold, wherean amplitude of the second voltage is greater than an amplitude of thefirst voltage. The second voltage may be applied for a time period or anumber of repetitions that is based on the fatigue threshold of theferroelectric memory cell 105-b. In some cases, memory controller 140-amay increment or reset counter 605, or it may start or reset timer 610.

In some cases, some memory cells 105 may be accessed while other memorycells 105 are being recovered. For example, memory controller 140-a mayalso perform the fatigue recovery operation on a first ferroelectricmemory cell 105 of the plurality of ferroelectric memory cells 105-b andperform an access operation on a second ferroelectric memory cell 105 ofthe plurality during the fatigue recovery operation of the firstferroelectric memory cell 105.

In some examples, after determining that the at least one memory cell105-b has reached its fatigue threshold, the logic state of theferroelectric memory cell 105-b may be stored in another memory cell105, for example, in cache 615, because the recovery operation may bedestructive. After caching, the second voltage may be applied to theferroelectric memory cell 105-b. After the recovery operation, a logicstate of the other memory cell 105 (in cache 615) may be determined andit may be written to the recovered ferroelectric memory cell 105-b. Suchan operation may be performed for multiple memory cells 105.

In some examples, the recovery operation may be applied to multiplememory cells 105. For example, memory cells 105-b may include multiplerows, where each row includes a plate line 210 in electroniccommunication with multiple ferroelectric memory cells 105. The secondvoltage may be applied to at least one plate line 210, where the secondvoltage is applied to each ferroelectric memory cell 105 that is inelectronic communication with the plate line 210. In some cases, thesecond voltage may be applied to multiple plate lines 210.

FIG. 7 shows a block diagram 700 of a memory array 100-b that supportsrecovery of fatigued ferroelectric memory cells in accordance withvarious embodiments of the present disclosure. Memory array 100-b may bereferred to as an electronic memory apparatus or an electronic circuitand may include memory controller 140-b and memory cell 105-c, which maybe examples of memory controller 140 and memory cell 105 described withreference to FIGS. 1, 2, and 6. Memory controller 140-b may includecounter 605-a and timer 610-a, which may be examples of a counter 605and timer 610 with reference to FIG. 6. Memory controller 140-b alsoincludes biasing component 710 and timing component 715 and may operatememory array 100-b as described in FIGS. 1-6. Memory controller 140-bmay be in electronic communication with word line 110-b, digit line115-b, sense component 125-b, and plate line 210-a, which may beexamples of word line 110, digit line 115, sense component 125, andplate line 210 described with reference to FIG. 1 or 2. Memory array100-b may also include reference component 720, latch 725, and terminals730, where terminals 730 may be in electronic communication with bus735. The components of memory array 100-b may be in electroniccommunication with each other and may perform the functions describedwith reference to FIGS. 1-6. In some cases, reference component 720,sense component 125-b, and latch 725 may be components of memorycontroller 140-b.

Memory controller 140-b may be configured to activate word line 110-b,plate line 210-a, or digit line 115-b by applying voltages to thosevarious nodes. For example, biasing component 710 may be configured toapply a voltage to operate memory cell 105-c to read, write, or performrecovery operations as described above. In some cases, memory controller140-b may include a row decoder, column decoder, or both, as describedwith reference to FIG. 1. This may enable memory controller 140-b toaccess one or more memory cells 105. Biasing component 710 may alsoprovide a voltage source for reference component 720 in order togenerate a reference signal for sense component 125-b. Additionally,biasing component 710 may provide voltages for the operation of sensecomponent 125-b.

Memory array 100-b may include multiple ferroelectric memory cells105-c. Memory array 100-b may also include a plurality of conductiveterminals 730 that are in electronic communication with the memoryarray. In some examples, terminals 730 may include a recovery terminalthat supports a fatigue recovery operation of the ferroelectric memorycells 105-c. In some examples, the fatigue recovery operation mayinclude a recovery voltage applied to at least one ferroelectric memorycell 105-c based on a determination that ferroelectric memory cell 105-chas reached a fatigue threshold. In some examples, an amplitude of therecovery voltage may be greater than an amplitude of an access voltage,and the recovery voltage may be applied for a time period or a number ofrepetitions that is based on the fatigue threshold.

In some cases, terminals 730 may include a first power terminal inelectronic communication with a first voltage supply. The first voltagemay be used for applying the access voltage and may be applicable toferroelectric memory cell 105-c of memory array 100-b during an accessoperation of ferroelectric memory cell 105-c. Terminals 730 may alsoinclude a second power terminal in electronic communication with asecond voltage supply. The second voltage may be used for applying therecovery voltage and may be applicable to ferroelectric memory cell105-c during the fatigue recovery operation of ferroelectric memory cell105-c.

In some examples, the fatigue recovery operation may include receiving acommand to initiate the fatigue recovery operation of the ferroelectricmemory cells 105-c via the recovery terminal. For example, anotherelement of the device may direct the memory array to perform the fatiguerecovery operation. In some cases, the command may indicate which memorycells should be recovered (e.g., the command may contain the addressesof the memory cells to be recovered). In some cases, the command toinitiate the fatigue recovery operation may be received from acontroller. The command may be received at certain intervals or it maybe triggered by a particular event, such as refresh operation or thelike.

In some cases, the fatigue recovery operation may include parametersthat are programmable by a user or third party, such as a devicemanufacturer that manufactures a device that includes the memory array.Such programmable parameters may include the time period or the numberof repetitions of the second voltage, or the amplitude of the secondvoltage. Other programmable parameters may include the threshold timeperiod in which the fatigue threshold is reached, the threshold numberof access cycles in which the fatigue threshold is reached, or the testwindow size. In some cases, programmable parameters may includedetermining which events the fatigue recovery operation may be applied,such as when the device powers on, powers off, or is connected to anexternal power supply.

In some cases, memory controller 140-b may perform its operations usingtiming component 715. For example, timing component 715 may control thetiming of the various word line selections or plate biasing, includingtiming for switching and voltage application to perform the memoryfunctions, such as reading, writing, and recovery, discussed herein. Insome cases, timing component 715 may control the operations of biasingcomponent 710.

Reference component 720 may include various components to generate areference signal for sense component 125-b. Reference component 720 mayinclude circuitry configured to produce a reference signal. In somecases, reference component 720 may be other ferroelectric memory cells105. In some examples, reference component 720 may be configured tooutput a voltage with a value between the two sense voltages, asdescribed with reference to FIG. 3. Or reference component 720 may bedesigned to output a virtual ground voltage (i.e., approximately 0V).

Sense component 125-b may compare a signal from memory cell 105-e(through digit line 115-b) with a reference signal from referencecomponent 720. Upon determining the logic state, the sense component maythen store the output in latch 725, where it may be used in accordancewith the operations of an electronic device that memory array 100-b is apart. In other cases, sense component 125-b may determine that memorycell 105-c has reached a fatigue threshold, where the fatigue thresholdof ferroelectric memory cell 105-c may be based on a relationshipbetween the charge stored in the ferroelectric memory cell 105-c and acapacitance of digit line 115-b in electronic communication with theferroelectric memory cell 105-c and sense component 125-b.

FIG. 8 illustrates a system 800 that supports recovery of fatiguedferroelectric memory cells in accordance with various embodiments of thepresent disclosure. System 800 includes a device 805, which may be orinclude a printed circuit board to connect or physically support variouscomponents. Device 805 includes a memory array 100-d, which may be anexample of memory array 100 described with reference to FIGS. 1, 6, and7. Memory array 100-c may contain memory controller 140-c and memorycell(s) 105-d, which may be examples of memory controller 140 describedwith reference to FIGS. 1, 6, and 7, and memory cells 105 described withreference to FIGS. 1, 2, 6, and 7. Device 805 includes bus 735-a, whichmay be an example of a bus 735 with reference to FIG. 7. Device 805 mayalso include a processor 810, BIOS component 815, peripheralcomponent(s) 820, and input/output control component 825. The componentsof device 805 may be in electronic communication with one anotherthrough bus 735-a.

Processor 810 may be configured to operate memory array 100-a throughmemory controller 140-c. In some cases, processor 810 may perform thefunctions of memory controller 140 described with reference to FIGS. 1and 7. In other cases, memory controller 140-c may be integrated intoprocessor 810. Processor 810 may be a general-purpose processor, adigital signal processor (DSP), an application-specific integratedcircuit (ASIC), a field-programmable gate array (FPGA) or otherprogrammable logic device, discrete gate or transistor logic, discretehardware components, or it may be a combination of these types ofcomponents, and processor 810 may perform various functions describedherein, including recovery of cycling wear-out in ferroelectricmemories. Processor 810 may, for example, be configured to executecomputer-readable instructions stored in memory array 100-c to causedevice 805 perform various functions or tasks. In some cases, processor810 may send a command to memory array 100-c in order to access itsstored contents. Memory array 100-c may suspend a recovery operationbased on the command and may resume the recovery operation afterperforming the commanded operation.

BIOS component 815 may be a software component that includes a basicinput/output system (BIOS) operated as firmware, which may initializeand run various hardware components of system 800. BIOS component 815may also manage data flow between processor 810 and the variouscomponents, e.g., peripheral components 820, input/output controlcomponent 825, etc. BIOS component 815 may include a program or softwarestored in read-only memory (ROM), flash memory, or any othernon-volatile memory.

Peripheral component(s) 820 may be any input or output device, or aninterface for such devices, that is integrated into device 805. Examplesmay include disk controllers, sound controller, graphics controller,Ethernet controller, modem, universal serial bus (USB) controller, aserial or parallel port, or peripheral card slots, such as peripheralcomponent interconnect (PCI) or accelerated graphics port (AGP) slots.

Input/output control component 825 may manage data communication betweenprocessor 810 and peripheral component(s) 820, input devices 835, oroutput devices 840. Input/output control component 825 may also manageperipherals not integrated into device 805. In some cases, input/outputcontrol component 825 may represent a physical connection or port to theexternal peripheral.

Input 835 may represent a device or signal external to device 805 thatprovides input to device 805 or its components. This may include a userinterface or interface with or between other devices. In some cases,input 835 may be a peripheral that interfaces with device 805 viaperipheral component(s) 820 or may be managed by input/output controlcomponent 825. In some examples, input 835 may enable the user toprogram the recovery operations.

Output 840 may represent a device or signal external to device 805configured to receive output from device 805 or any of its components.Examples of output 840 may include a display, audio speakers, a printingdevice, another processor or printed circuit board, etc. In some cases,output 840 may be a peripheral that interfaces with device 805 viaperipheral component(s) 820 or may be managed by input/output controlcomponent 825.

The components of memory controller 140-c, device 805, and memory array100-c may be made up of circuitry designed to carry out their functions.This may include various circuit elements, for example, conductivelines, transistors, capacitors, inductors, resistors, amplifiers, orother active or inactive elements, configured to carry out the functionsdescribed herein.

FIG. 9 shows a flowchart illustrating a method 900 for recovery offatigued ferroelectric memory cells in accordance with variousembodiments of the present disclosure. The operations of method 900 maybe implemented by a memory array 100, as described with reference toFIGS. 1-7. For example, the operations of method 900 may be performed bya memory controller 140 as described with reference to FIGS. 1, 6, 7,and 8. In some examples, a memory controller 140 may execute a set ofcodes to control the functional elements of the memory array 100 toperform the functions described below. Additionally or alternatively,the memory controller 140 may perform the functions described belowusing special-purpose hardware.

At block 905, the method may include applying a first voltage to aferroelectric memory cell during each cycle of a plurality of accesscycles, as described with reference to FIGS. 1-8. In some cases,applying the first voltage to access the ferroelectric memory cellincludes reading or writing the ferroelectric memory cell. In certainexamples, the operations of block 905 may be performed or facilitated bythe memory controller 140, as described with reference to FIGS. 1, 6, 7,and 8.

At block 910, the method may include determining that the ferroelectricmemory cell has reached a fatigue threshold based on applying the firstvoltage for the plurality of access cycles, as described with referenceto FIGS. 1-8. In some examples, determining that the ferroelectricmemory cell has reached the fatigue threshold includes detecting atleast one of a timer exceeding a threshold time period, where thethreshold time period is based at least in part on a time to reach thefatigue threshold; a counter exceeding a threshold number of counts,where the counter is incremented for each access cycle of the pluralityof access cycles; a failure to sense the ferroelectric memory cell basedon applying a test access voltage with an amplitude less than anamplitude of the first voltage; a failure to sense the ferroelectricmemory cell based on a test sense window that is less than a sensewindow used for each cycle of the plurality of access cycles, where thetest sense window is based at least in part on a reference voltage; oran error correction code detecting a corrupted logic value of theferroelectric memory cell. In some examples, determining that theferroelectric memory cell has reached the fatigue threshold includesreceiving a command to perform a fatigue recovery operation. In certainexamples, the operations of block 910 may be performed or facilitated bythe memory controller 140, as described with reference to FIGS. 1, 6, 7,and 8, or the counter 605 or timer 610, as described with reference toFIGS. 6 and 7.

At block 915, the method may include applying a second voltage to theferroelectric memory cell based at least in part on determining that theferroelectric memory cell has reached the fatigue threshold, where anamplitude of the second voltage is greater than an amplitude of thefirst voltage, and where the second voltage is applied for a time periodor a number of repetitions that is based at least in part on the fatiguethreshold of the ferroelectric memory cell, as described with referenceto FIGS. 1-8.

In some examples, the second voltage includes at least one of aplurality of bipolar voltage pulses, a plurality of unipolar voltagepulses, or a voltage with a constant amplitude. The time period or thenumber of repetitions of the second voltage may be based on theamplitude of the second voltage. In some examples, the method mayinclude incrementing a counter each time the second voltage is appliedor starting a timer while a voltage with a constant amplitude isapplied.

In some cases, applying the second voltage for the time period or thenumber of repetitions includes applying the second voltage duringmultiple instances of an event, where a subset of the time period or asubset of the number of repetitions is associated with each instance ofthe event. In some examples, the method may include suspending theapplication of the second voltage based on receiving a request to accessthe memory array and resuming the application of the second voltagebased on completing the request.

In some examples of the method, the memory array may be an element of adevice, and applying the second voltage to the ferroelectric memory cellmay include applying the second voltage to the ferroelectric memory cellduring an event that includes least one of: the device powering on, thedevice powering off, or the device being connected to an external powersupply. In certain examples, the operations of block 915 may beperformed or facilitated by the memory controller 140, as described withreference to FIGS. 1, 6, 7, and 8.

In some examples, the method may include determining a logic state ofthe ferroelectric memory cell based on determining that theferroelectric memory cell has reached the fatigue threshold and storingthe logic state of the ferroelectric memory cell in another memory cell,where the second voltage is applied to the ferroelectric memory cellafter storing the logic state of the ferroelectric memory cell in theother memory cell. The method may include determining a logic state ofthe other memory cell after applying the second voltage and writing thelogic state of the other memory cell to the ferroelectric memory cell.

The method may include applying the second voltage during a refreshoperation that restores a logic state to the ferroelectric memory cell,where the logic state is determined based at least in part on applyingthe first voltage.

In some examples, the memory array comprises a plurality of rows, whereeach row of the plurality of rows comprises a plate line in electroniccommunication with a plurality of ferroelectric memory cells. Applyingthe second voltage may include applying the second voltage to at leastone plate line, where the second voltage is applied to eachferroelectric memory cell that is in electronic communication with theplate line. In some examples, applying the second voltage to the plateline includes applying the second voltage to a plurality of plate lines.

In some examples of the method, the fatigue threshold of theferroelectric memory cell may be based on a relationship between thecharge stored in the ferroelectric memory cell and a capacitance of adigit line in electronic communication with the ferroelectric memorycell and a sense component. In other examples, the fatigue threshold ofthe ferroelectric memory cell may be based on a total number of accesscycles in which the ferroelectric memory cell reaches a remnantpolarization threshold.

The method may also include determining that a charge stored in theferroelectric memory cell after applying the second voltage is greaterthan a charge stored in the ferroelectric memory cell before applyingthe second voltage, where the determination is based on a sense voltageproduced by the ferroelectric memory cell.

Thus, method 900 may be a method for operating a memory array to providefor recovery of fatigued ferroelectric memory cells. It should be notedthat method 900 describes possible implementations, and the operationsand steps may be rearranged or otherwise modified such that otherimplementations are possible.

The description herein provides examples, and is not limiting of thescope, applicability, or examples set forth in the claims. Changes maybe made in the function and arrangement of elements discussed withoutdeparting from the scope of the disclosure. Various examples may omit,substitute, or add various procedures or components as appropriate.Also, features described with respect to some examples may be combinedin other examples.

The description set forth herein, in connection with the appendeddrawings, describes example configurations and does not represent allthe examples that may be implemented or that are within the scope of theclaims. The terms “example,” “exemplary,” and “embodiment,” as usedherein, mean “serving as an example, instance, or illustration,” and not“preferred” or “advantageous over other examples.” The detaileddescription includes specific details for the purpose of providing anunderstanding of the described techniques. These techniques, however,may be practiced without these specific details. In some instances,well-known structures and devices are shown in block diagram form inorder to avoid obscuring the concepts of the described examples.

In the appended figures, similar components or features may have thesame reference label. Further, various components of the same type maybe distinguished by following the reference label by a dash and a secondlabel that distinguishes among the similar components. When the firstreference label is used in the specification, the description isapplicable to any one of the similar components having the same firstreference label irrespective of the second reference label.

Information and signals described herein may be represented using any ofa variety of different technologies and techniques. For example, data,instructions, commands, information, signals, bits, symbols, and chipsthat may be referenced throughout the above description may berepresented by voltages, currents, electromagnetic waves, magneticfields or particles, optical fields or particles, or any combinationthereof. Some drawings may illustrate signals as a single signal;however, it will be understood by a person of ordinary skill in the artthat the signal may represent a bus of signals, where the bus may have avariety of bit widths.

As used herein, the term “virtual ground” refers to a node of anelectrical circuit that is held at a voltage of approximately zero volts(0V) but that is not directly connected with ground. Accordingly, thevoltage of a virtual ground may temporarily fluctuate and return toapproximately 0V at steady state. A virtual ground may be implementedusing various electronic circuit elements, such as a voltage dividerconsisting of operational amplifiers and resistors. Otherimplementations are also possible. “Virtual grounding” or “virtuallygrounded” means connected to approximately 0V.

The term “electronic communication” refers to a relationship betweencomponents that supports electron flow between the components. This mayinclude a direct connection between components or may includeintermediate components. Components in electronic communication may beactively exchanging electrons or signals (e.g., in an energized circuit)or may not be actively exchanging electrons or signals (e.g., in ade-energized circuit) but may be configured and operable to exchangeelectrons or signals upon a circuit being energized. By way of example,two components physically connected via a switch (e.g., a transistor)are in electronic communication regardless of the state of the switch(i.e., open or closed).

The term “isolated” refers to a relationship between components in whichelectrons are not presently capable of flowing between them; componentsare isolated from each other if there is an open circuit between them.For example, two components physically connected by a switch may beisolated from each other when the switch is open.

As used herein, the term “shorting” refers to a relationship betweencomponents in which a conductive path is established between thecomponents via the activation of a single intermediary component betweenthe two components in question. For example, a first component shortedto a second component may exchange electrons with the second componentwhen a switch between the two components is closed. Thus, shorting maybe a dynamic operation that enables the flow of charge betweencomponents (or lines) that are in electronic communication.

The terms “amplitude” and “magnitude” as used in relation to physicalvalues, signals, or quantities, may be synonymous.

The devices discussed herein, including memory array 100, may be formedon a semiconductor substrate, such as silicon, germanium,silicon-germanium alloy, gallium arsenide, gallium nitride, etc. In somecases, the substrate is a semiconductor wafer. In other cases, thesubstrate may be a silicon-on-insulator (SOI) substrate, such assilicon-on-glass (SOG) or silicon-on-sapphire (SOP), or epitaxial layersof semiconductor materials on another substrate. The conductivity of thesubstrate, or sub-regions of the substrate, may be controlled throughdoping using various chemical species including, but not limited to,phosphorous, boron, or arsenic. Doping may be performed during theinitial formation or growth of the substrate, by ion-implantation, or byany other doping means.

A transistor or transistors discussed herein may represent afield-effect transistor (FET) and comprise a three terminal deviceincluding a source, drain, and gate. The terminals may be connected toother electronic elements through conductive materials, e.g., metals.The source and drain may be conductive and may comprise a heavily-doped,e.g., degenerate, semiconductor region. The source and drain may beseparated by a lightly-doped semiconductor region or channel. If thechannel is n-type (i.e., majority carriers are electrons), then the FETmay be referred to as a n-type FET. If the channel is p-type (i.e.,majority carriers are holes), then the FET may be referred to as ap-type FET. The channel may be capped by an insulating gate oxide. Thechannel conductivity may be controlled by applying a voltage to thegate. For example, applying a positive voltage or negative voltage to ann-type FET or a p-type FET, respectively, may result in the channelbecoming conductive. A transistor may be “on” or “activated” when avoltage greater than or equal to the transistor's threshold voltage isapplied to the transistor gate. The transistor may be “off” or“deactivated” when a voltage less than the transistor's thresholdvoltage is applied to the transistor gate.

The various illustrative blocks, components, and modules described inconnection with the disclosure herein may be implemented or performedwith a general-purpose processor, a DSP, an ASIC, an FPGA or otherprogrammable logic device, discrete gate or transistor logic, discretehardware components, or any combination thereof designed to perform thefunctions described herein. A general-purpose processor may be amicroprocessor, but in the alternative, the processor may be anyconventional processor, controller, microcontroller, or state machine. Aprocessor may also be implemented as a combination of computing devices(e.g., a combination of a DSP and a microprocessor, multiplemicroprocessors, one or more microprocessors in conjunction with a DSPcore, or any other such configuration).

The functions described herein may be implemented in hardware, softwareexecuted by a processor, firmware, or any combination thereof. Ifimplemented in software executed by a processor, the functions may bestored on or transmitted over as one or more instructions or code on acomputer-readable medium. Other examples and implementations are withinthe scope of the disclosure and appended claims. For example, due to thenature of software, functions described above can be implemented usingsoftware executed by a processor, hardware, firmware, hardwiring, orcombinations of any of these. Features implementing functions may alsobe physically located at various positions, including being distributedsuch that portions of functions are implemented at different physicallocations. Also, as used herein, including in the claims, “or” as usedin a list of items (for example, a list of items prefaced by a phrasesuch as “at least one of” or “one or more of”) indicates an inclusivelist such that, for example, a list of at least one of A, B, or C meansA or B or C or AB or AC or BC or ABC (i.e., A and B and C).

Computer-readable media includes both non-transitory computer storagemedia and communication media including any medium that facilitatestransfer of a computer program from one place to another. Anon-transitory storage medium may be any available medium that can beaccessed by a general purpose or special purpose computer. By way ofexample, and not limitation, non-transitory computer-readable media cancomprise RAM, ROM, electrically erasable programmable read only memory(EEPROM), compact disk (CD) ROM or other optical disk storage, magneticdisk storage or other magnetic storage devices, or any othernon-transitory medium that can be used to carry or store desired programcode means in the form of instructions or data structures and that canbe accessed by a general-purpose or special-purpose computer, or ageneral-purpose or special-purpose processor.

Also, any connection is properly termed a computer-readable medium. Forexample, if the software is transmitted from a website, server, or otherremote source using a coaxial cable, fiber optic cable, twisted pair,digital subscriber line (DSL), or wireless technologies such asinfrared, radio, and microwave, then the coaxial cable, fiber opticcable, twisted pair, digital subscriber line (DSL), or wirelesstechnologies such as infrared, radio, and microwave are included in thedefinition of medium. Disk and disc, as used herein, include CD, laserdisc, optical disc, digital versatile disc (DVD), floppy disk andBlu-ray disc where disks usually reproduce data magnetically, whilediscs reproduce data optically with lasers. Combinations of the aboveare also included within the scope of computer-readable media.

The description herein is provided to enable a person skilled in the artto make or use the disclosure. Various modifications to the disclosurewill be readily apparent to those skilled in the art, and the genericprinciples defined herein may be applied to other variations withoutdeparting from the scope of the disclosure. Thus, the disclosure is notto be limited to the examples and designs described herein but is to beaccorded the broadest scope consistent with the principles and novelfeatures disclosed herein.

What is claimed is:
 1. A method, comprising: determining that aparameter of a memory cell of a memory array has satisfied a fatiguethreshold; and applying a recovery voltage to the memory cell based atleast in part on determining that the parameter of the memory cell hassatisfied the fatigue threshold.
 2. The method of claim 1, furthercomprising: applying a test voltage having an amplitude less than anamplitude of an access voltage, wherein determining that the parameterof the memory cell has satisfied the fatigue threshold is based at leastin part on applying the test voltage.
 3. The method of claim 1, furthercomprising: determining a failure to sense a value stored on the memorycell based at least in part on a test sense window being smaller than asense window used during an access operation, wherein determining thatthe parameter of the memory cell has satisfied the fatigue threshold isbased at least in part on detecting the failure to sense the value. 4.The method of claim 1, further comprising: applying error correction toan unknown logic value, wherein determining that the parameter of thememory cell has satisfied the fatigue threshold is based at least inpart on applying the error correction.
 5. The method of claim 1, furthercomprising: determining that a timer satisfies a threshold, whereindetermining that the parameter of the memory cell has satisfied thefatigue threshold is based at least in part on determining that thetimer satisfies the threshold.
 6. The method of claim 1, furthercomprising: counting a number of access operations performed on thememory cell in which the memory cell has reached a remnant polarizationthreshold; and determining that the number of access operationssatisfies a threshold, wherein determining that the parameter of thememory cell has satisfied the fatigue threshold is based at least inpart on determining that the number of access operations satisfies thethreshold.
 7. The method of claim 1, wherein the recovery voltage isdifferent from an access voltage applied during an access operation ofthe memory cell.
 8. The method of claim 1, wherein the memory cell is aferroelectric memory cell.
 9. The method of claim 1, further comprising:storing a logic state of the memory cell based at least in part ondetermining that the parameter of the memory cell has satisfied thefatigue threshold; and re-writing the logic state to the memory cellafter applying the recovery voltage.
 10. The method of claim 1, whereina duration of the application of the recovery voltage, or a number ofrepetitions of the application of the recovery voltage, or both arebased at least in part on the recovery voltage.
 11. The method of claim1, wherein the recovery voltage comprises at least one of: a pluralityof bipolar voltage pulses; a plurality of unipolar voltage pulses; or avoltage with a constant amplitude.
 12. A circuit, comprising: a memoryarray including a plurality of memory cells; and a memory controlleroperable to: initiate a fatigue recovery operation for a memory cell ofthe plurality of memory cells; determine that a parameter of the memorycell has satisfied a fatigue threshold; and apply a recovery voltage tothe memory cell of the plurality of memory cells based at least in parton the determination that the parameter has satisfied the fatiguethreshold.
 13. The circuit of claim 12, wherein the recovery voltage isdifferent from an access voltage of an access operation.
 14. The circuitof claim 12, wherein the memory controller is further operable to: applya test voltage having different from an access voltage of an accessoperation, wherein the determination that the parameter of the memorycell has satisfied the fatigue threshold is based at least in part onapplying the test voltage.
 15. The circuit of claim 12, wherein thememory controller is further operable to: determine a failure to sense avalue stored on the memory cell based at least in part on a test sensewindow being smaller than a sense window used during an accessoperation, wherein the determination that the parameter of the memorycell has satisfied the fatigue threshold is based at least in part ondetermining the failure to sense the value.
 16. The circuit of claim 12,wherein the memory controller is further operable to: apply errorcorrection to an unknown logic value, wherein the determination that theparameter of the memory cell has satisfied the fatigue threshold isbased at least in part on applying the error correction.
 17. The circuitof claim 12, wherein the memory controller is further operable to:determine that a timer satisfies a threshold, wherein the determinationthat the parameter of the memory cell has satisfied the fatiguethreshold is based at least in part on determining that the timersatisfies the threshold.
 18. A method, comprising: determining that amemory cell has satisfied a fatigue threshold based at least in part ona number of access operations that have been performed on the memorycell; and applying a recovery voltage to the memory cell based at leastin part on determining that the memory cell has satisfied the fatiguethreshold.
 19. The method of claim 18, further comprising: pulsing therecovery voltage until a number of repetitions of the application of therecovery voltage satisfies a recovery threshold, wherein applying therecovery voltage is based at least in part on the pulsing of therecovery voltage.
 20. The method of claim 18, further comprising:incrementing a counter each time an access operation is performed on thememory cell; and determining that the counter satisfies a countthreshold, wherein determining that the memory cell has satisfied thefatigue threshold is based at least in part on determining that thecounter satisfies the count threshold.